There are many well-known methods of mounting electronic components to a substrate. One method is the conventional "reflow soldering" process used for attaching terminations of surface mount components. In the conventional reflow soldering process the terminations of the surface mount components have a thin pre-tin solder coating and are attached during a manufacturing process to rectangular mounting pads etched onto a substrate. The process comprises printing a solder paste through a stencil having apertures matching the size and location of the mounting pads, placing the surface mount component terminations on top of the solder paste in alignment with the mounting pads therefor, and passing the substrate and surface mount components through a reflow solder oven for heating the pre-tin solder coating and solder paste to a liquefied state to attach the terminations to the mounting pads.
Another conventional method for mounting a device on a substrate is known as clad or solid solder deposition (SSD), a process which effectively eliminates the solder paste screening process described above. In this process, a specific amount of pre-applied and solidified solder is deposited on the pads prior to component placement. During component attachment, a predetermined amount of tacky solder flux is deposited on the clad portion. Later, the surface mount component is placed on the clad and the pre-applied solder is reflown to attach the component to the substrate.
Disadvantageously, during the conventional reflow soldering and SSD processes termination attachment defects occur because the terminations of the surface mount components do not always remain aligned with the mounting pads. Errors in initial placement of the surface mount components, vibrations from equipment used to move the substrate through a manufacturing area, thermal gradient, and general handling also can cause misalignment.
Unfortunately, the rectangular mounting pad geometry used in the conventional process are only partially effective in correcting any misalignment that occurs. Other pad geometries, such as circular, oval and/or, tri-oval have also been used to correct the misalignment problem.
However, the trend of electronic devices towards smaller sizes requiring micro-miniature components tends to increase defect rates even further in the conventional reflow soldering process. This is because the defect rate due to misalignment increases as the terminations, mounting pads, and alignment tolerances become smaller.
Thus, what is needed is a better way of mounting surface mount components to corresponding mounting pads on a substrate. A pad arrangement that can minimize resulting component misalignment and reduce attachment defects on microminiature components is highly desired.